#include <unistd.h>  
#include <stdio.h>  
#include <fcntl.h>  
#include <linux/fb.h>  
#include <sys/mman.h>  
inline static unsigned short int make16color(unsigned char r, unsigned char g, unsigned char b)  
{  
    return (  
 (((r >> 3) & 31) << 11) |  
 (((g >> 2) & 63) << 5)  |  
  ((b >> 3) & 31)        );  
}  
int main() {  
    int fbfd = 0;  
    struct fb_var_screeninfo vinfo;  
    struct fb_fix_screeninfo finfo;  
    long int screensize = 0;  
 char *fbp = 0;  
    int x = 0, y = 0;  
    int guage_height = 20, step = 10;  
    long int location = 0;  
    // Open the file for reading and writing  
    fbfd = open("/dev/graphics/fb0", O_RDWR);  
    if (!fbfd) {  
        printf("Error: cannot open framebuffer device.\n");  
        exit(1);  
    }  
    printf("The framebuffer device was opened successfully.\n");  
    // Get fixed screen information  
    if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {  
        printf("Error reading fixed information.\n");  
        exit(2);  
    }  
    // Get variable screen information  
    if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {  
        printf("Error reading variable information.\n");  
        exit(3);  
    }  
 printf("sizeof(unsigned short) = %d\n", sizeof(unsigned short));  
    printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel );  
 printf("xoffset:%d, yoffset:%d, line_length: %d\n", vinfo.xoffset, vinfo.yoffset, finfo.line_length );  
    // Figure out the size of the screen in bytes  
    screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;;  
    // Map the device to memory  
    fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,  
                       fbfd, 0);  
    if ((int)fbp == -1) {  
        printf("Error: failed to map framebuffer device to memory.\n");  
        exit(4);  
    }  
    printf("The framebuffer device was mapped to memory successfully.\n");  
 //set to black color first  
 memset(fbp, 0, screensize);  
    //draw rectangle  
    y = (vinfo.yres - guage_height) / 2 - 2;       // Where we are going to put the pixel  
    for (x = step - 2; x < vinfo.xres - step + 2; x++) {  
        location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +  
                       (y+vinfo.yoffset) * finfo.line_length;  
        *((unsigned short int*)(fbp + location)) = 255;  
    }  
    y = (vinfo.yres + guage_height) / 2 + 2;       // Where we are going to put the pixel  
    for (x = step - 2; x < vinfo.xres - step + 2; x++) {  
        location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +  
                       (y+vinfo.yoffset) * finfo.line_length;  
        *((unsigned short int*)(fbp + location)) = 255;  
    }  
    x = step - 2;  
    for (y = (vinfo.yres - guage_height) / 2 - 2; y < (vinfo.yres + guage_height) / 2 + 2; y++) {  
        location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +  
                       (y+vinfo.yoffset) * finfo.line_length;  
        *((unsigned short int*)(fbp + location)) = 255;  
    }  
    x = vinfo.xres - step + 2;  
    for (y = (vinfo.yres - guage_height) / 2 - 2; y < (vinfo.yres + guage_height) / 2 + 2; y++) {  
        location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +  
                       (y+vinfo.yoffset) * finfo.line_length;  
        *((unsigned short int*)(fbp + location)) = 255;  
    }  
    // Figure out where in memory to put the pixel  
    for ( x = step; x < vinfo.xres - step; x++ ) {  
        for ( y = (vinfo.yres - guage_height) / 2; y < (vinfo.yres + guage_height) / 2; y++ ) {  
            location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +  
                       (y+vinfo.yoffset) * finfo.line_length;  
            if ( vinfo.bits_per_pixel == 32 ) {  
                *(fbp + location) = 100;        // Some blue  
                *(fbp + location + 1) = 15+(x-100)/2;     // A little green  
                *(fbp + location + 2) = 200-(y-100)/5;    // A lot of red  
                *(fbp + location + 3) = 0;      // No transparency  
            } else { //assume 16bpp  
                unsigned char b = 255 * x / (vinfo.xres - step);  
                unsigned char g = 255;     // (x - 100)/6 A little green  
                unsigned char r = 255;    // A lot of red  
                unsigned short int t = make16color(r, g, b);  
                *((unsigned short int*)(fbp + location)) = t;  
            }  
        }  
  //printf("x = %d, temp = %d\n", x, temp);  
        //sleep to see it  
        usleep(200);  
    }  
    //clean framebuffer  
    munmap(fbp, screensize);  
    close(fbfd);  
    return 0;  
}
